In typical commercial reproduction apparatus (electrostatographic copier/duplicators, printers, or the like), a latent image charge pattern is formed on a uniformly charged charge-retentive or photoconductive member having dielectric characteristics (hereinafter referred to as the dielectric support member). Pigmented marking particles are attracted to the latent image charge pattern to develop such image on the dielectric support member. A receiver member, such as a sheet of paper, transparency or other medium, is then brought into contact with the dielectric support member, and an electric field applied to transfer the marking particle developed image to the receiver member from the dielectric support member. After transfer, the receiver member bearing the transferred image is transported away from the dielectric support member, and the image is fixed (fused) to the receiver member by heat and pressure to form a permanent reproduction thereon.
One type of fuser assembly for typical electrographic reproduction apparatus includes at least one heated roller, having an aluminum core and an elastomeric cover layer, and at least one pressure roller in nip relation with the heated roller. The fuser assembly rollers are rotated to transport a receiver member, bearing a marking particle image, through the nip between the rollers. The pigmented marking particles of the transferred image on the surface of the receiver member soften and become tacky in the heat. Under the pressure, the softened tacky marking particles attach to each other and are partially imbibed into the interstices of the fibers at the surface of the receiver member. Accordingly, upon cooling, the marking particle image is permanently fixed to the receiver member.
A typical roller fuser assembly of the prior art is shown in FIG. 1. The roller fuser assembly 10 includes a fuser roller 12, a pressure roller 14, and other necessary sub-systems and components (not shown). The roller 12 (or both rollers 12 and 14) is heated internally (for example by lamps 16, 18), and/or externally for example by heat rollers 20 (as shown in FIG. 2), to pre-set temperatures. When fusing prints on receiver members S, the rollers 12 and 14 are pressed together to form a nip, and rotation of the rollers drive prints through the nip. In the nip, heat energy stored in the fuser roller 12 is transferred to the prints, and heats up and melts the toner image carried by the receiver member. Under the temperature and pressure, the toner is fixed on the receiver member.
The configuration of the fuser roller 12 can greatly affect the receiver member release characteristics and heat transfer of the fuser. Generally the fuser roller 12 has a metal core 22, a base cushion 24, and a thin release topcoat 26. A thicker base cushion makes release geometry in the nip area more favorable for the receiver member to be released from the fuser roller 12, but makes the heat more difficult to transfer from the core 22 to the outer surface of the topcoat 26.
To preserve the favorable release geometry and improve the heat transfer characteristics, some kind of external heaters are often used. FIG. 2 shows a fuser with one or two roller external heaters 20, with heating lamps 30 inside. The metal external heating rollers 20 have high thermal conductivity and can transfer higher amount of heat than other external heating methodologies, such as radiation heating. They are also simple, less expensive, and present less potential fire hazards. However, since the external heating rollers 20 usually have small diameter, it is difficult to provided a large nip between an external heating roller and a fuser roller. This limits the heat transfer rate between an external heating roller 20 and a fuser roller 12. Furthermore, a high force between the external heating roller 20 and the fuser roller 12, may cause wear and damage to the fuser roller topcoat 26.